A nozzle unit in accordance with the above is generally known in the field of abrasive water jet machining. Devices for abrasive water jet machining typically operate at an ambient pressure substantially equal to atmospheric pressure. The water jet, which is virtually free of any solids, is jetted into a mixing chamber at a pressure of well above 1 kbar. A dry abrasive material is kept at atmospheric pressure and due to the jet pump mechanism in the mixing chamber, the abrasive particles are sucked into the mixing chamber through the abrasive particle inlet.
In the field of drilling holes into geological earth formations, an abrasive water jet system including a nozzle unit with a jet pump mechanism can be used for drilling a hole, see for example WO 02/34653. However, the conditions in this field are substantially different from the field of atmospheric abrasive jet machining since the ambient pressure is well above atmospheric pressure and increases with about 1 bar per 10 meters depth.
In the case of the atmospheric abrasive water jet machining systems, air is sucked into the mixing chamber together with the abrasive particles. This air flow into the nozzle unit may generate cavitation that can limit the transfer of kinetic energy from the water jet to the abrasive material. Consequently, the efficiency of the nozzle unit, which is based on this kinetic energy transfer, is limited by the cavitation.
Another important source of cavitation may stem from turbulence in and around the jet stream. Pressure fluctuations in the turbulence locally include pressures below the vapour pressure of the carrier fluid, which possibly causes vaporization, the creation of gas bubbles, and cavitation.
There is a desire for a nozzle unit that is able to impart at an as high as possible efficiency kinetic energy to abrasive particles at an as low as possible consumption rate of abrasive particles so that the nozzle unit can be used within a limited space available in a typical bore hole in a geological earth formation.
International application WO-A 91/12930 mentions an efficiency reduction of conventional nozzle units when applied in increased ambient pressure conditions, and reports the construction of a nozzle unit that allows for a relatively easy modification of the mixing chamber length. This measure corrects the nozzle design for the increase in jet divergence caused by the gradual decrease of a cavitation shield around the jet with ambient pressure.
U.S. Pat. No. 4,555,872 describes a nozzle apparatus in accordance with the preamble for generating an abrasive fluid jet stream having material cutting capabilities for objects at atmospheric pressure. A first nozzle is provided with an orifice plate of sapphire, having a cone-shaped orifice of which the smallest flow opening has a diameter of approximately 0.5 mm (0.020 inch). Herewith an extremely high pressure-drop is achievable at a low flow rate. A second nozzle downstream of the first nozzle is provided in the form of a tapered flow shaping cone, of which the smallest flow opening has a diameter of approximately 1.5 mm (0.060 inch).
It has been found that none of the prior art nozzle units described above is capable of delivering a satisfactory abrasive jet stream in a high pressure surrounding such as is typically encountered when drilling holes into geological earth formations, taking into consideration special boundary conditions that apply.